Bi-fuel engine for vehicle

ABSTRACT

A bi-fuel engine is equipped with an engine body, a plurality of branched intake pipes, a surge tank, and an air cleaner. Each of the branched intake pipes has a length including a slant portion and a bend. The bend is located on the side of the engine body and connects at an end to the slant portion and at the other end to a downstream end of the branched intake pipe. The slant portion extends from the bend to the upstream end of the branched intake pipe obliquely to a horizontal direction of the engine body so as to define between the surge tank and an upper portion of the engine body a space which is large enough to permit a cylinder head cover to be installed to and uninstalled from the upper portion of the engine body.

CROSS REFERENCE TO RELATED DOCUMENT

The present application claims the benefit of priority of JapanesePatent Application No. 2011-251707 filed on Nov. 17, 2011, thedisclosure of which is totally incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates generally to a bi-fuel engine for vehicleswhich runs on two types of fuel: one is liquid fuel, and the other isgas fuel.

2. Background Art

Bi-fuel engines designed to run on two types of fuel: one being liquidfuel such as gasoline, and the other being gas fuel such as compressednatural gas (CNG), are known as internal combustion engine forautomotive vehicles. A typical one of such bi-fuel engines is equippedwith a gas fuel injection device disposed on a cylinder head covermounted on the top of an engine body and gas fuel hoses connecting gasfuel injectors of the gas fuel injection device and an intake manifoldtogether. The gas fuel hoses extend beneath a liquid fuel injectiondevice.

For example, Japanese Patent First Publication No. 2010-242559 teachessuch a type of bi-fuel engine.

The above structure in which the gas fuel injection device and the gasfuel hoses are arranged on the top of the cylinder head cover, however,faces drawbacks, as discussed below. The removal of the cylinder headcover from the engine body requires steps of dismounting the liquid fuelinjection device, disconnecting the intake manifold from the gas fuelinjection device, uninstalling the gas fuel injection device, anddisconnecting the gas fuel pipes from the intake manifold. The assemblyor disassembly of the engine is, therefore, quite inconvenient.

Additionally, the gas fuel injection device is located close to theintake manifold. The total length of the gas fuel hoses connectingbetween the gas fuel injection device and the intake manifold is,therefore, short. It is, thus, difficult to bend the gas fuel hoses tolay out and joint them to the engine.

Moreover, the gas fuel injection device is mounted over the top of thecylinder head cover, so that it greatly protrudes outside the cylinderhead cover. The exertion of external force on the vehicle may, thus,result in physical interference of the gas fuel injection device withperipheral devices.

Further, when an air cleaner is disposed above the engine, it mayinterfere with the gas fuel injection device which projects greatly fromthe cylinder head cover. The air cleaner is, thus, subjected to therestriction on volume thereof.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a bi-fuel enginefor vehicles which is designed to ensure an increased volume of an aircleaner, be easy to assemble, and/or protect a gas fuel injection devicefrom an external force applied to the vehicle.

According to one aspect of the invention, there is provided a bi-fuelengine which is to be mounted in a vehicle such as an automobile. Thebi-fuel engine comprises: (a) an engine body which has a cylinder headcover disposed detachably on an upper portion thereof; (b) a pluralityof branched intake pipes joined at downstream ends thereof to a side ofthe engine body in communication with an intake path in the engine body;(c) a surge tank to which an upstream end of each of the branched intakepipes is coupled; (d) an air cleaner which communicates with the surgetank; (e) a liquid fuel injection device which works to inject a liquidfuel into the engine body; (f) a plurality of gas fuel hoses each ofwhich is joined at a first end thereof to a portion of one of thebranched intake pipes which is located near the downstream end; and (g)a gas fuel injection device equipped with a plurality of gas fuelinjectors each of which connects with a second end of one of the gasfuel hoses.

Each of the branched intake pipes has a length including a slant portionand a bend. The bend is located laterally to the engine body and has afirst end leading to the slant portion and a second end leading to thedownstream end of the branched intake pipe. The slant portion extendsfrom the first end of the bend to the upstream end of the branchedintake pipe at a given angle to a horizontal direction of the enginebody so as to create between the surge tank and the upper portion of theengine body a space which is large enough to permit the cylinder headcover to be installed to and uninstalled from the upper portion of theengine body.

In the preferred mode of the embodiment, the air cleaner is disposedabove the slant portions of the branched intake pipes. The gas fuelinjection device is disposed in a clearance between the surge tank andthe air cleaner. The gas fuel injectors are located near the upstreamends of the branched intake pipes. The gas fuel hoses extend along thebranched intake pipes, respectively. The liquid fuel injection device isdisposed in a space defined between the engine body and the branchedintake pipes.

Each of the gas fuel injectors is so inclined at a given angle to thehorizontal direction of the engine body as to have a head to which oneof the gas fuel hoses is joined and which faces the bend of one of thebranched intake pipes.

The gas fuel injectors and the gas fuel hoses are disposed alternatelyin a direction in which the branched intake pipes are arrayed.

The gas fuel injection device and the air cleaner are secured to thesurge tank.

The engine body is so designed as to be mounted in a vehicle with thebranched intake pipes facing a rear of the vehicle.

The above described structure improves the ease of assembly of thebi-fuel engine, permits the air cleaner to have an increased volume, andserves to protect the gas fuel injection device physically when anexternal force is applied to the bi-fuel engine.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given hereinbelow and from the accompanying drawings of thepreferred embodiment of the invention, which, however, should not betaken to limit the invention to the specific embodiment but is for thepurpose of explanation and understanding only.

In the drawings:

FIG. 1 is a side view which illustrates a bi-fuel engine according to anembodiment of the invention;

FIG. 2 is a back view which illustrates a rear side of the bi-fuelengine of FIG. 1;

FIG. 3 is a plan view of the bi-fuel engine of FIG. 1; and

FIG. 4 is a partial plan view which illustrates highlights of thebi-fuel engine of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, wherein like reference numbers refer to likeparts in several views, particularly to FIG. 1, there is shown a bi-fuelengine 1 according to an embodiment of the invention. Note that eachview schematically illustrates parts of the bi-fuel engine 1 regardlessof their true sizes or ratios among them for the sake of simplicity ofillustration.

Outline Structure of Engine

The bi-fuel engine 1 is, as clearly illustrated in FIG. 1, mountedwithin an engine compartment 3 formed in the front 2 of an automotivevehicle. The bi-fuel engine 1 consists essentially of an engine body 4,an intake manifold 5, an exhaust manifold 6, an air cleaner 7, a liquidfuel injection device 8, a plurality of gas fuel hoses (i.e., hollowtubes) 9, and a gas fuel injection device 11. The gas fuel injectiondevice 11 is equipped with a plurality of gas fuel injectors 10B.

Engine Body

The engine body 4 includes a cylinder block 12, a cylinder head 13mounted on the cylinder block 12, a cylinder head cover 14 disposed onthe cylinder head 13, a lower crankcase 15 formed below the cylinderblock 12, and an oil pan 16 (also called a sump) arranged below thelower crankcase 15.

The lower crankcase 15 supports or bears a crankshaft 17 rotatably. Thecylinder block 12 has formed therein a plurality of cylinders (notshown) within which pistons (not shown) reciprocate vertically. Thevertical reciprocating motion of each piston is transmitted andconverted into rotational motion of the crankshaft 17.

The cylinder head 13 has formed therein a plurality of intake paths 18through which air is brought into the cylinders of the cylinder block 12The cylinder head 13 also has formed therein a plurality of exhaustpaths 19 through which exhaust gas is discharged from the cylinders.Additionally, the cylinder head 13 has intake valves and exhaust valves(not shown) mounted above the cylinders to open or close the intake andexhaust paths 18 and 19.

A valve train (not shown) is mounted on the top of the cylinder head 13.The valve train works to control the operation of the intake and exhaustvalves at a given timing. The cylinder head cover 14 is secured to thetop of the cylinder head 13 detachably using a fastening mechanism suchas bolts. The maintenance such as repair or adjustment of the valvetrain and other parts is usually achieved by removing the cylinder headcover 14 from the cylinder head 13.

Intake Manifold

The intake manifold 5, as illustrated in FIG. 1, consists of a pluralityof branched intake pipes 51 and a surge tank 52. The surge tank 52 islaid above the cylinder head cover 14 at a distance L away therefrom.The distance L is a vertical interval between a lower surface of thesurge tank 52 and an upper surface of the cylinder head cover 14 and soselected as to permit the cylinder head cover 14 to be removed from orinstalled to the cylinder head 13 without any physical interference withthe surge tank 52. In other words, the distance L is so determined as tocreate between the surge tank 52 and the cylinder head cover 14 (i.e.,an upper portion of the engine body 4) a working space S1 which has avolume large enough to enable, for example, an assembly worker todismount the cylinder head cover 14 from the cylinder head 13 withoutany physical interference with the surge tank 52. The distance L may bedefined along a line extending vertically from the center of a traversesection of the surge tank 52.

The removal of the cylinder head cover 14, as can be seen from FIG. 1,enables the valve train (including, for example, a camshaft) mounted onthe cylinder head 13 to be visually perceived or refurbished ormaintained from a direction, as indicated by an arrow A.

The branched intake pipes 51, as clearly illustrated in FIG. 4,communicate at upstream ends 51A thereof with outlets of the surge tank52. The branched intake pipes 51 extend vertically, as viewed from thedrawing, and are arrayed at a given interval D1 away from each other ina lengthwise direction of the surge tank 52 (i.e., a lateral directionin the drawing). The interval D1 is a distance between longitudinalcenter lines of adjacent two of the branched intake pipes 51. Thebranched intake pipes 51, as can be seen in FIGS. 1 and 2, also connectat downstream ends 51B thereof to inlets which are formed in a side wallof the cylinder head 13 and communicate with the intake paths 18,respectively. In this embodiment, the branched intake pipes 51 of theintake manifold 5 are so disposed as to extend from the engine body 4backward in the engine compartment 3.

Each of the branched intake pipes 51 has a given length made up of anupstream portion, a downstream portion, and a middle portion. The middleportion is curved (which will also be referred to as a bend 51C below)and located closer to the downstream end 51B than to the upstream end51A. The bend 51C of each of the branched intake pipes 51 is locatedlaterally to the engine body 4 (i.e., the engine head 13 and thecylinder head cover 14) and made to direct the branched intake pipe 51,as extending horizontally (i.e., the longitudinal direction of thevehicle) from the side wall of the cylinder head 13, obliquely upward tothe surge tank 52. In other words, the bend 51C is so formed as todefine a substantially U-shape of the branched intake pipe 51 andconnect the downstream portion and the upstream portion of the branchedintake pipe 51 together. The downstream portion extends substantiallyhorizontally (i.e., backward of the vehicle) from the inlet in the sidewall of the cylinder head 13. The upstream portion extends from behindthe cylinder head 13 obliquely forward to the outlet of the surge tank52. The upstream portion of the branched intake pipe 51 is a slantportion 51D, as illustrated in FIG. 1, extending from the upstream end51A obliquely downward in the backward direction of the vehicle. Inother words, the slant portion 51D extends at a given angle (greaterthan zero) to the horizontal direction of the engine body 4 so as tocreate the working space S1 between the surge tank 52 and the upperportion of the engine body 4.

As apparent from the above discussion, each of the branched intake pipes51 does not extend straight from the side of the cylinder head 13 to thesurge tank 52, but bypasses the side of the cylinder head 13. Theexhaust manifold 6 is, as clearly illustrated in FIGS. 1 and 3, joinedto the front side of the cylinder head 13 which is opposed to the rearside to which the intake manifold 5 is joined.

Liquid Fuel Injection Device

The liquid fuel injection device 8 is, as illustrated in FIGS. 1 and 2,disposed among an array of the branched intake pipes 51, the cylinderhead 13, and the cylinder head cover 14. The liquid fuel injectiondevice 8 includes a liquid fuel delivery pipe 8A and a plurality ofliquid fuel injectors 8B which are joined to the liquid fuel deliverypipe 8A and arrayed at a given interval away from each other.

The liquid fuel delivery pipe 8A extends in a direction in which thebranched intake pipes 51 are arrayed, that is, in the lateral direction(i.e., the widthwise direction) of the vehicle when the bi-fuel engine 1is mounted in the engine compartment 3. Each of the liquid fuelinjectors 8B is mounted in the wall of the cylinder head 13 and works tospray liquid fuel such as gasoline into a corresponding one of theintake paths 18.

The liquid fuel delivery pipe 8A of the liquid fuel injection device 8is, as can be seen in FIG. 2, joined to a liquid fuel tank 8C in whichthe liquid fuel is stored. When a controller such as an engineelectronic control unit installed in the vehicle enters a liquid fuelinjection mode, it outputs a control signal to each of the liquid fuelinjectors 8B to spray the liquid fuel, as delivered from the liquid fueltank 8C, into the combustion chambers of the bi-fuel engine 1.

As is clear from the above discussion, the liquid fuel injection device8 is disposed in a chamber defined by the array of branched intake pipes51, the cylinder head 13, and the cylinder head cover 14. It isadvisable that the size of the chamber be set by selecting the curvature(e.g., the radius of curvature) of the bend 51C of each of the branchedintake pipes 51.

Air Cleaner

The air cleaner 7 which communicates with the surge tank 52 is, asillustrated in FIGS. 1 and 2, disposed just above the slant portions 51Dof the branched intake pipes 51. The air cleaner 7 extends over anoverall length of the slant portions 51D. The slant portions 51D, asdescribed above, extend obliquely downward to the bends 51C so as tocreate a space above the bends 51C which is greater in height enough toaccommodate the air cleaner 7. The air cleaner 7 has a mount plate 7Aformed on an edge thereof close to the surge tank 52. The mount plate 7Ais fixed to the surge tank 52 through bolts 20.

The air cleaner 7 is, unlike the prior art structure, disposed withoutany interference with the gas fuel injection device 11. The size orvolume of the air cleaner 7 may, therefore, be increased more than thatof the conventional air cleaner.

Gas Fuel Injection Device

The gas fuel injection device 11 is, as illustrated in FIGS. 1, 2, and4, equipped with a gas fuel delivery pipe 10A, gas fuel injectors 10B,and a gas fuel tank 10C. The gas fuel delivery pipe 10A is, as can beseen in FIG. 4, disposed parallel to a line extending through theupstream ends 51A of the branched intake pipes 51 connecting with thesurge tank 52. The gas fuel delivery pipe 10A is joined to the gas fueltank 10C through a pipe (not shown). The gas fuel tank 10C stores thegas fuel such as compressed natural gas (CNG).

The gas fuel injectors 10B are, as illustrated in FIG. 4, joined to thegas fuel delivery pipe 10A. The gas fuel injectors 10B are arrayed at agiven interval D2 away from each other along the length of the gas fueldelivery pipe 10A (i.e., in the lateral direction of the vehicle) incommunication with outlets of the gas fuel delivery pipe 10A. Theinterval D2 between adjacent two of the gas fuel injectors 10B issubstantially identical with the interval D1 between the upstream ends51A of adjacent two of the branched intake pipes 51. The gas fuelinjectors 10B and the upstream ends 51A of the branched intake pipes 51are arranged alternately in a direction in which the branched intakepipes 51 are arrayed (i.e., the lateral direction of the vehicle).

The gas fuel injection device 11 equipped with the gas fuel injectors10B and the gas fuel delivery pipe 10A is, as can be seen in FIG. 1,disposed in a clearance S2 between the air cleaner 7 and the surge tank52. Each of the gas fuel injectors 10B is, as clearly illustrated inFIGS. 1 and 4, located near the upstream end 51A of one of the branchedintake pipes 51 between adjacent two or on the side of one of theupstream ends 51A (i.e., the slant portions 51D). Such a layout of theslant portions 51D of the branched intake pipes 51 avoids strong directinterference of the gas fuel injectors 10B which are lower in rigidityin the gas fuel injection device 11 with parts of the engine 1 orperipheral devices mounted in the vehicle when an external force acts onthe vehicle. Specifically, the engine 1 serves to protect the gas fuelinjection device 11 physically.

Each of the gas fuel injectors 10B, as illustrated in FIG. 1, has a topend (i.e., a head) working as an outlet 10D connected to one of the gasfuel hoses 9 and is oriented obliquely relative to the verticaldirection, that is, inclined at a given angle to the horizontaldirection of the engine body 4 to have the outlet 10D facing the bend51C of a corresponding one of the branched intake pipe 51. In otherwords, the outlet 10D of each of the gas fuel injectors 10B has a centerline or axis extending through or near the bend 51C of the branchedintake pipe 51.

The gas fuel injection device 11, as illustrated in FIGS. 1 and 3, has amounting member 10E made of, for example, a bracket. The mounting member10E is secured to the surge tank 52 using a bolt 21 to retain the gasfuel delivery pipe 10A and the gas fuel injectors 10B together.

Each of the gas fuel hoses 9 is, as illustrated in FIGS. 1, 2, and 4,joined at an end 9A thereof to a portion of a corresponding one of thebranched intake pipes 51 near the downstream end 51B and at an end 9Bthereof to the outlet 10D of a corresponding one of the gas fuelinjectors 10B. The gas fuel hoses 9, as illustrated in FIG. 1, extendalong the branched intake pipes 51.

When it is required to run the engine 1 on the gas fuel, the controllerof the vehicle, as described above, outputs the control signal to eachof the gas fuel injectors 10B of the gas fuel injection device 11 tospray the gas fuel into one of the branched intake pipes 51.

As described above, the engine 1 is so designed that the intake manifold5 and the air cleaner 7 do not physically interfere with the gas fuelinjection device 11, thereby permitting the branched intake pipes 51 ofthe intake manifold 5 to be increased in length thereof and alsofacilitating installation of the air cleaner 7 of an increased volume ascompared with the conventional structure. It is also possible toincrease the volume of the air cleaner 7 in order to decrease theresistance to the suction of air thereinto.

The gas fuel injection device 11 is, as described above, disposedbetween the surge tank 52 and the air cleaner 7, thus enabling themounting member 10E of the gas fuel injection device 11 and the mountplate 7A of the air cleaner 7 to be both fixed to the surge tank 52. Thesurge tank 52 occupies an upper portion of the engine 1, thus improvingthe ease of installation of the air cleaner 7 and the gas fuel injectiondevice 11.

The location of the gas fuel injection device 11 between the surge tank52 and the air cleaner 7 also eliminates the need for an assembly workerto dismount the gas fuel injection device 11 (i.e., the gas fueldelivery pipe 10A and the gas fuel injectors 10B) and the gas fuel hoses9 when the cylinder head cover 14 is removed from the engine body 4.This facilitates the ease with which the cylinder head cover 14 isinstall in or uninstall from the working space Si between the upperportion of the engine body 4 and the surge tank 52, thus improving theease of assembly of the engine 1 further.

An increase in overall length of the gas fuel hoses 9, as compared withthe conventional structure, is achieved by laying the gas fuel hoses 9between the gas fuel injectors 10B and the downstream ends 51B of thebranched intake pipes 51 along the increased length of the branchedintake pipes 51. This facilitates the ease with which the assemblyworker bends or curves the gas fuel hoses 9 to couple them with the gasfuel injectors 10B and the downstream ends 51B of the branched intakepipes 51.

The increase in overall length of the gas fuel hoses 9 also facilitatesthe ease with which the gas fuel hoses 9 are laid out around orinstalled to the engine body 4 within the narrow engine compartment 3,thus enhancing the ease of assembly of the engine 1 further.

The increase in overall length of the gas fuel hoses 9 permits them tobe made from material which is relatively low in flexibility withoutsacrificing the ease with which the gas fuel hoses 9 are laid out aroundor installed to the engine body 4, i.e., the assembly workability of theengine 1. This also allows the gas fuel hoses 9 to be formed by materialwhich is higher in durability and rigidity than conventional gas fuelhoses.

Each of the gas fuel injectors 10B is, as described above, laid near theupstream end 51A of one of the branched intake pipes 51 between adjacenttwo of the upstream ends 51A (i.e., the slant portions 51D) or on theside of one of the upstream ends 51A. Such a layout of the gas fuelinjectors 10B in relation to the slant portions 51D of the branchedintake pipes 51 avoids strong direct interference of the gas fuelinjectors 10B which are lower in rigidity in the gas fuel injectiondevice 11 with parts of the engine 1 or peripheral devices mounted inthe vehicle when an external force acts on the vehicle. Specifically,the engine 1 serves to protect the gas fuel injection device 11physically.

The use of the mount plate 7A in securing the air cleaner 7 to the surgetank 52 through the bolt 20 facilitates the ease with which the aircleaner 7 is installed to or uninstalled from the surge tank 52.

Each of the gas fuel injectors 10B is so inclined at a given angle tothe vertical direction as to have the outlet (i.e., the head) 10D whichis joined to one of the gas fuel hoses 9 and faces the bend 51C of acorresponding one of the branched intake pipes 51. This enables theassembly worker to visually perceive the outlet 10D of the gas fuelinjector 10 from the direction, as indicated by an arrow B in FIG. 1,when the gas fuel hose 9 is coupled to the outlet 10D, thus resulting inan improvement of ease of installation of the gas fuel hose 9 to theengine 1. Additionally, the smooth coupling of the gas fuel hose 9 tothe outlet 10D of the gas fuel injector 10B is achieved by moving anupper portion of the gas fuel hose 9 along the upper side of the slantportion 51D of the branched intake pipe 51. This also enhances the easeof installation of the gas fuel hose 9 to the engine 1, i.e., the easeof assembly of the engine 1.

The bracket-like mounting member 10E of the gas fuel injection device 11which holds the gas fuel delivery pipe 10A and the gas fuel injectors10B together is, as described above, attached to the surge tank 52through the bolt 21, thus facilitating the ease of installation of thegas fuel injection device 11 to the surge tank 52.

The inclined orientation of the branched intake pipes 51 (i.e., theslant portions 51D), as described above, develops the space large enoughto accommodate the air cleaner 7. The branched intake pipes 51 arearrayed at a given interval away from each other to create gaps whichserve to protect the gas fuel injection device 11 (i.e., the gas fuelinjectors 10B) physically.

While the present invention has been disclosed in terms of the preferredembodiment in order to facilitate better understanding thereof, itshould be appreciated that the invention can be embodied in various wayswithout departing from the principle of the invention. Therefore, theinvention should be understood to include all possible embodiments andmodifications to the shown embodiment which can be embodied withoutdeparting from the principle of the invention as set forth in theappended claims.

For instance, the gas fuel hoses 9 are laid along the sides of thebranched intake pipes 51, but however may be disposed slightly closer tothe engine body 4 than to the sides of the branched intake pipes 51,that is, pass through a space between the branched intake pipes 51 andthe engine body 4.

The engine 1 is mounted in the engine compartment 3 with the intakemanifold 5 located on a rear side of the engine 1 which faces the rearof the vehicle, but may be disposed in another orientation in the enginecompartment 3.

The liquid fuel injection device 8 is disposed in a space between thebranched intake pipes 51 and the engine body 4, but may be arrangedoutside the space.

What is claimed is:
 1. A bi-fuel engine for a vehicle comprising: an engine body which has a cylinder head cover disposed detachably on an upper portion thereof; a plurality of branched intake pipes joined at downstream ends thereof to a side of the engine body in communication with an intake path in the engine body; a surge tank to which an upstream end of each of the branched intake pipes is coupled; an air cleaner which communicates with the surge tank; a liquid fuel injection device which works to inject a liquid fuel into the engine body; a plurality of gas fuel hoses each of which is joined at a first end thereof to a portion of one of the branched intake pipes which is located near the downstream end; and a gas fuel injection device equipped with a plurality of gas fuel injectors each of which connects with a second end of one of the gas fuel hoses, wherein each of the branched intake pipes has a length including a slant portion and a bend, the bend being located laterally to the engine body and having a first end leading to the slant portion and a second end leading to the downstream end of the branched intake pipe, the slant portion extending from the first end of the bend to the upstream end of the branched intake pipe at a given angle to a horizontal direction of the engine body so as to create between the surge tank and the upper portion of the engine body a space which is large enough to permit the cylinder head cover to be installed to and uninstalled from the upper portion of the engine body.
 2. A bi-fuel engine as set forth in claim 1, wherein the air cleaner is disposed above the slant portions of the branched intake pipes, wherein the gas fuel injection device is disposed in a clearance between the surge tank and the air cleaner, the gas fuel injectors being located near the upstream ends of the branched intake pipes, wherein the gas fuel hoses extend along the branched intake pipes, respectively, and wherein the liquid fuel injection device is disposed in a space defined between the engine body and the branched intake pipes.
 3. A bi-fuel engine as set forth in claim 1, wherein each of the gas fuel injectors is so inclined at a given angle to the horizontal direction of the engine body as to have a head to which one of the gas fuel hoses is joined and which faces the bend of one of the branched intake pipes.
 4. A bi-fuel engine as set forth in claim 1, wherein the gas fuel injectors and the gas fuel hoses are disposed alternately in a direction in which the branched intake pipes are arrayed.
 5. A bi-fuel engine as set forth in claim 1, wherein the gas fuel injection device and the air cleaner are secured to the surge tank.
 6. A bi-fuel engine as set forth in claim 1, wherein the engine body is so designed as to be mounted in a vehicle with the branched intake pipes facing a rear of the vehicle. 